Various forms of variable dampers have been proposed for use in wheel suspension systems for the purposes of improving the ride quality and achieving a motion stability of the vehicle. In a common conventional variable damper, a rotary valve is incorporated in the piston for varying an effective area of an orifice that communicates the two chambers on either side of the piston with each other, and such a rotary valve is typically actuated mechanically by using a suitable actuator. More recently, it has become more common to use magneto-rheological fluid for the actuating fluid of the damper, and control the viscosity of the fluid by supplying corresponding electric current to a magnetic valve which is incorporated in the piston. According to such an arrangement, the overall structure can be simplified, and the response property of the damper can be improved. See U.S. Pat. No. 6,260,675, for instance.
The piston of the damper disclosed in U.S. Pat. No. 6,260,675 comprises a cylindrical inner yoke, a coil wound around the outer periphery of the inner yoke, a pair of end plates placed on either axial end of the inner yoke, and a cylindrical outer yoke coaxially surrounding the inner yoke and end plates. The inner yoke and outer yoke are both made of magnetic material, and are retained in a spaced apart relationship by the end plates so as to define an annular flow passage between them. The end plates typically consist of disks made of non-magnetic material, and are each provided with a plurality of arcuate slots communicating with the annular passage, an annular recess for engaging a projection on the corresponding axial end of the inner yoke and an annular groove for engaging a ring that secures the inner end of the piston rod to the piston. The inner yoke, end plates and outer yoke are securely attached to one another by crimping each axial end of the outer yoke against the peripheral edge of the corresponding end plate.
In such a damper, it is desired to minimize the overall size of the damper. It is also desired to maximize the dynamic range of the damping force that can be produced by the damper. In maximizing the dynamic range of the damping force, it is beneficial to minimize the damping force that is produced when the coil is de-energized. It can be accomplished in various different ways, but reducing the axial length of the piston is one of the most effective ways.
In this regard, the damper disclosed in U.S. Pat. No. 6,260,675 requires a pair of end plates that have a relatively large thickness, and this prevents the compact design of the damper and the minimization of the flow resistance of the flow passage extending through the piston. Furthermore, the damper disclosed in U.S. Pat. No. 6,260,675 requires the outer yoke to be crimped onto the end plates, and this requires special tooling in the manufacturing line and makes the servicing of the damper extremely difficult.
U.S. Pat. No. 6,637,556 and U.S. Pat. No. 6,318,519 disclose dampers in which the piston is formed with axial grooves along the outer circumferential surface thereof to the end of minimizing the damping force when the coil is de-energized and optimizing the damping force in relation to the piston speed.
Another desirable attribute of a damper is a high responsiveness which can be accomplished by minimizing the inductance of the coil. In the conventional dampers mentioned above, because the coil is located immediately adjacent to the inner circumferential surface of the gap or flow passage in the piston, the coil is given with a high inductance, and this was found to be detrimental to achieving a favorable response of the damper.